Reason is because in the new videobuf2 functions didn't look
very clear how to support USERPTR, and run them through the
custom "evenly separated physical pages" test, to be able
to use other's driver MMAP'd address, passed as USERPTR to us.

- Remove custom mmap function to use ISS_PREALLOC space:

Reason is to stick with current available frameworks, without
depending on a major unaccepted patchset for now (like CMA).